Method for enhanced recovery of oil from oil reservoirs

ABSTRACT

The present invention provides a method for recovering oil from a subterranean reservoir using waterflooding, wherein the flooding fluid used in the waterflooding process comprises water and one or more ionic polyvinyl alcohol copolymers. The use of one or more ionic polyvinyl alcohol copolymers is expected to increase the recovery of oil by improving both the oil/water mobility ratio and the sweep efficiency in reservoirs with a high degree of heterogeneity.

FIELD OF THE INVENTION

The present invention relates to a process for recovering crude oil fromoil reservoirs using a flooding fluid comprising water and one or moreionic polyvinyl alcohol copolymers.

BACKGROUND OF THE INVENTION

In the recovery of oil from oil-bearing reservoirs, it is typicallypossible to recover only minor portions of the original oil in place byprimary recovery methods which utilize only the natural forces presentin the reservoir. Thus a variety of supplemental techniques have beendeveloped and used to increase oil recovery. A commonly used secondarytechnique is waterflooding which involves injection of water into theoil reservoir. As the water moves through the reservoir, it displacesoil therein to one or more production wells through which the oil isrecovered.

One problem encountered with waterflooding operations is the relativelypoor sweep efficiency of the water, i.e., the water can channel throughcertain portions of the reservoir as it travels from the injectionwell(s) to the production well(s), thereby bypassing other portions ofthe reservoir. Poor sweep efficiency may be due, for example, todifferences in the mobility of the water versus that of the oil, andpermeability variations within the reservoir which encourage flowthrough some portions of the reservoir and not others.

Various enhanced oil recovery techniques have been used to improve sweepefficiency. One such technique involves increasing the viscosity of thewater using non-biodegradable thickening agents such as polyvinylaromatic sulfonates as described in U.S. Pat. No. 3,085,063. U.S. Pat.No. 4,678,032 describes a method for treating a subterranean formationby injecting a polymer solution which includes an essentially non-ionicpolymer selected from the group consisting ofpoly(vinylalcohol-co-vinylcarboxyl) andpoly(vinylalcohol-co-vinylether), which is crosslinked with a transitionmetal selected from Groups IIIa-VIa, VII, and Ib-Vb of the PeriodicTable to form a gel. Such cross linked polymer systems require acrosslinker and monomer/polymer in at the current concentration, at thesame location and at a temperature appropriate for crosslinking to occurdeep in the reservoir. Such conditions are difficult to achieve inreality. Furthermore, the crosslinkers are often expensive for theamounts required and the polymer system are not readily dissolvable incold ocean water where it may be required, for example, on a oilproduction platform at sea.

There is therefore a need for a method to improve sweep efficiency usingcost-effective, biodegradable materials that exhibit shear-thinningproperties and thus exhibit lower viscosity during injection andincreased viscosity in the oil reservoir. The present invention providessuch a method. Another aspect of this invention is to provide a polymerfor this application that is readily soluble in cold sea water.Application of such a polymer would be an advantage for operations on anoil platform at sea (for example, the North Sea), since the water usedfor flooding the oil reservoir is cold sea water and there are no orlimited resources on typical platforms to heat up sea water to helpdissolve the polymer.

SUMMARY OF THE INVENTION

The present invention relates to the recovery of oil from a subterraneanreservoir using waterflooding. In one aspect, the present inventionprovides a method for recovering oil from a reservoir by waterflooding,comprising:

(a) introducing an aqueous flooding fluid into the reservoir, wherein atleast one portion of said flooding fluid comprises one or more ionicpolyvinyl alcohol copolymers, said one or more ionic polyvinyl alcoholcopolymers comprising:

-   -   (i) one or more anionic comonomers present at a total of about 1        to about 5 mol % relative to the combined moles of vinyl alcohol        and vinyl acetate;    -   (ii) optionally, one or more non-ionic comonomers present at        about 0 to about 7 mol % relative to the combined moles of vinyl        alcohol and vinyl acetate;        wherein the total monomers in (i) and (ii) are present at less        than or equal to about 8 mol %; wherein the hydrolysis level of        the one or more ionic polyvinyl alcohol copolymers is greater        than or equal to about 90%; and wherein the one or more ionic        polyvinyl alcohol copolymers dissolves substantially completely        in water at about 25° C. within about 14 hours; and

(b) displacing oil in the reservoir with said flooding fluid into one ormore production wells, whereby the oil is recoverable.

In another aspect, the present invention provides a flooding fluid foruse in water flooding operations.

DETAILED DESCRIPTION

The present invention relates to the recovery of oil from a subterraneanreservoir using waterflooding. Waterflooding is a technique that iscommonly used for secondary oil recovery from oil reservoirs. Accordingto this technique, water is injected through one or more wells into thereservoir, and as the water moves through the reservoir, it acts todisplace oil therein to one or more production wells through which theoil is recovered. According to the present invention, the efficacy ofwaterflooding is improved through the use of one or more ionic polyvinylalcohol copolymers. Thus, in one aspect, the present invention providesa flooding fluid for use in waterflooding operations comprising water,wherein at least one portion of said water comprises one or more ionicpolyvinyl alcohol copolymers, said one or more ionic polyvinyl alcoholcopolymers. The copolymers could contain anionic comonomers (such as C₁to C₄ straight-chain or branched alkyl esters of acrylic or methacrylicacid) from about 0 to about 7 mol % (or less than 8 mol %) relative tothe combined moles of vinyl alcohol and vinyl acetate. In the presentinvention, the hydrolysis level of the one or more ionic polyvinylalcohol copolymers is greater than or equal to about 90%; and one ormore ionic polyvinyl alcohol copolymers dissolves greater than or equalto about 95%, in water at about 25° C. within about 14 hours.

The invention also discloses a method for recovering oil from areservoir by waterflooding, through introducing an aqueous floodingfluid into the reservoir. In one aspect, the flooding fluid comprisesone or more ionic polyvinyl alcohol copolymers and one or more ionicpolyvinyl alcohol copolymers to a total of about 1 to about 5 mol %relative to the combined moles of vinyl alcohol and vinyl acetate.

The following definitions are provided for the special terms andabbreviations used in this application:

As used herein, “shear thinning” refers to the reduction of viscosity ofa liquid (such as that portion of the flooding fluid comprising the oneor more ionic polyvinyl alcohol copolymers) under shear stress.“Viscosity” refers to the resistance of a liquid (such as water or oil)to flow.

The term “water” refers to water that can be supplied from any suitablesource, and can include, for example, sea water, brine, productionwater, water recovered from an underground aquifer, including thoseaquifers in contact with the oil, or surface water from a stream, river,pond or lake. As is known in the art, it may be necessary to removeparticulates from the water prior to injection into the one or morewells.

The term “mobility” refers to the ratio of the permeability to the flowof a liquid to the dynamic viscosity of said liquid (Boatright, KE,2002, Basic Petroleum Engineering Practices, 9.6; see also IntegratedPetroleum Management—A Team Approach, (A. Sattar and G. Thakurm,PennWell Books, Tulsa, Okla., 1994)).

The term “mobility ratio” is the mobility of the water ratioed to thatof the mobility of the oil. Mobilization of oil is enhanced from anunderground oil containing reservoir or rock when the mobility of theoil is more than the mobility of the water—that is this ratio is lessthan one—in this case the mobility ratio is considered favorable formobilizing oil. However, even if the mobility ratio is greater than onewhich may be the case more oil can be produced by thickening the waterand moving this ratio lower even if the ratio is not less than one.

The term “viscosity ratio” is defined as the ratio of the solutionviscosity measured at that temperature and at a shear rate of 1 sec⁻¹ tothe solution viscosity measured at that temperature and at a shear rateof 10 sec⁻¹

The term “production wells” refers to wells through which oil iswithdrawn from a reservoir. An oil reservoir or oil formation is asubsurface body of rock having sufficient porosity and permeability tostore and transmit oil.

The term “one or more ionic polyvinyl alcohol copolymers” refers to oneor more polyvinyl alcohol copolymers. Polyvinyl Alcohol (PVOH) ismanufactured commercially by polymerization of vinyl acetate monomer(VAM) to afford polyvinyl acetate (PVAc). The PVAc is thentransesterified—in most commercial processes with methanol, in whichcase it is also described as methanolysis—to yield PVOH and methylacetate. The % hydrolysis (or hydrolysis level) of the polymer isdefined as the molar amount of vinyl alcohol divided by the sum of themolar amount of vinyl alcohol plus the molar amount of vinyl acetate inthe polymer. In one aspect, the % hydrolysis is greater than or equal toabout 95%. In a more specific aspect, the % hydrolysis is greater thanor equal to about 98%. In an even more specific aspect, the % hydrolysisis greater than or equal to about 99%.

PVOH homopolymer that is >98% hydrolyzed (that is, less than 2% residualvinyl acetate) is not suitable for the present invention because it doesnot dissolve at practical temperatures, as it usually requirestemperatures in excess of 50° C. to dissolve. For the present inventionpolymers that dissolve in water at a temperature of less than about 25°C. are preferred. This can be accomplished by modifying PVOH to reducecrystallinity and/or increase hydrophilicity of the polymer. Thecrystallinity of the polymer can be reduced by carrying out thetransesterification of PVAc in such a manner as to not complete theconversion to PVOH and obtain a product that is conventionally known aspartially hydrolyzed PVOH (phPVOH). Commercial grades of 88% hydrolyzedphPVOH include Celvol™ 523 from Celanese (Dallas, Tex.) and KurarayPOVAL™ PVA 217 sold by Kuraray Co., Ltd. (Osaka, JP).Post-polymerization or post-copolymerization modifications of PVOH canreduce crystallinity and/or increase the hydrophilicity of the polymer.Post polymerization reactions have been reviewed (PolyvinylAlcohol-Developments; Finch, C. A., Ed.; John Wiley & Sons: New York,1992).

One known copolymerization method to increase hydrophilicity is tocopolymerize VAM with an ionic, acid-containing monomer such as acrylicacid as described in U.S. Pat. No. 4,885,105. After transesterificationof such a copolymer a hydrophilic carboxylic acid or carboxylic acidsalt remains. Inclusion of up to about 5 mol % comonomer can modify thepolymer solubility adequately for use in the present invention. Othersuitable acids include maleic acid, itaconic acid and methacrylic acid.One or more C₁ to C₄ straight-chain or branched alkyl monoesters ofitaconic and maleic acid can also be used. Another useful comonomer forimparting hydrophilicity is the sodium salt of2-acrylamido-2-methyl-1-propanesulfonate (AMPS). This sodium salt ofAMPS (or SAMPS, CAS# 5165-97-9) has increased tolerance of low pHenvironments and high salt concentrations. The preparation of PVOH/AMPScopolymers has been described (T. Moritani and J. Yamauchi, Polymer, 39,553-557, 1998 and U.S. Pat. No. 6,818,709). Other salt forms of AMPS canalso be used for the present invention, such as potassium, ammonium, andtetramethylammonium. Salts of AMPS can be included in an amount of about1 to about 5 mol %. In a more specific aspect, salts of AMPS can beincluded in an amount of from about 2 mol % to about 4 mol %. In an evenmore specific aspect, salts of AMPS can be included in an amount of fromabout 3 mol % to about 4 mol %. Copolymers with anionic groups haveadvantage over non-ionic, partially hydrolyzed PVOH because theygenerally dissolve faster.

Optionally, one or more nonionic comonomers can be copolymerized with atleast one ionic monomer. Examples include the C₁ to C₄ straight-chain orbranched alkyl esters of acrylic or methacrylic acid, synthesized asdescribed in U.S. Patent Publications 2005/0154120 and 2006/0035042.Methyl acrylate or methyl methacrylate are preferred, and methylacrylate is most preferred. Acrylate comonomers are known to formlactone rings with neighboring vinyl alcohol groups during thetransesterification process (Polyvinyl Alcohol-Developments; Finch, C.A., Ed.; John Wiley & Sons: New York, 1992). The combination of ionicmonomer and acrylate- or methacrylate-derived lactone reduces the amountof ionic monomer necessary for solubility, which has the advantages ofreducing the amount of caustic catalyst necessary fortransesterification, reducing water sensitivity of the solid polymer,and reducing the overall cost of the polymer. The lactone-containingpolymer can be optionally treated with a base such as sodium orpotassium hydroxide to form the ring-opened carboxylate form asdescribed in US 2007/0034575, in which case the monomer is consideredanionic. The acrylate or methacrylate ester can be used at about 0 mol %to about 7 mol %. In a more specific aspect, the acrylate ormethacrylate ester can be used at about 2 mol % to about 5 mol %. In aneven more specific aspect, the acrylate or methacrylate ester can beused at about 3 mol % to about 4 mol %. Additional non-ionic comonomersthat can be utilized include ethylene, acrylamide, and vinylpyrrolidone. It is recognized that during the transesterificationprocess, nonionic monomers with hydrolyzable groups such as acrylate ormethacrylate esters, or acrylamide, can undergo unintentional hydrolysisreactions depending on process conditions and the amount of water.

In one aspect, the one or more ionic polyvinyl alcohol copolymers has anaverage molecular weight greater than about 50,000 daltons as measuredby gel permeation chromatography. In a more specific aspect, the averagemolecular weight is greater than about 60,000 daltons. In an even morespecific aspect, the average molecular weight is greater than about70,000 daltons.

The present invention provides an advantage to existing technology inthat the one or more ionic polyvinyl alcohol copolymers as defined aboveare biodegradable (Chiellini, E., et al., Prog. Polym. Sci., 28:963-1014, 2003), and thus flooding fluid having these compounds can besafely released into the environment surrounding the oil recoveryoperation if necessary, or as an accidental release. In addition,flooding fluid comprising these compounds exhibits shear-thinningproperties, such that the solution exhibits low viscosity at high shearrates and increased viscosity at low shear rates.

The flooding fluid useful for waterflooding according to the presentinvention comprises water and one or more cold water soluble ionicpolyvinyl alcohol copolymers.

The flooding fluid useful for the waterflooding process of the inventioncomprises water, wherein at least a portion of said water comprises oneor more ionic polyvinyl alcohol copolymers. Thus, in one aspect, the oneor more ionic polyvinyl alcohol copolymers is added to a volume of waterand injected into the well(s), followed by the injection of additionalwater. This process can be repeated one or more times if necessary. Atthe injection well(s), which is under high pressure and high shear, therelative viscosity of at least one portion of the flooding fluidcomprising one or more ionic polyvinyl alcohol copolymers is low,whereas as at least one portion of the flooding fluid flows into thereservoir, the shear decreases and the relative viscosity increases. Theone or more ionic polyvinyl alcohol copolymers can also be added to theentire volume of flooding fluid, as long as the backpressure at theinjection well(s) does not become too high. As is known to those skilledin the art of oil recovery, the bottom well pressure of the injector cannot exceed the strength of the rock formation, otherwise formationdamage will occur at a given flow rate. Adjustments can be made byreducing the flow of the injection water, adding water to decreaseviscosity, or by adding water mixed with the one or more ionic polyvinylalcohol copolymers to increase viscosity in order to improve theefficacy of oil recovery.

The one or more ionic polyvinyl alcohol copolymers can be added as asolid powder to at least one portion of the flooding fluid. Theconcentration of the one or more ionic polyvinyl alcohol copolymers inat least one portion of the flooding fluid can be in the range of about0.007% to about 3% (weight of the one or more ionic polyvinyl alcoholcopolymers/total weight of the at least one portion of flooding fluidcomprising said one or more ionic polyvinyl alcohol copolymers). Inanother aspect, the concentration is in the range of about 0.1% to about1% (weight/weight).

In one aspect, one or more ionic polyvinyl alcohol copolymers is addedto the flooding fluid in order to increase the viscosity of at least oneportion of the water in the flooding fluid, thereby improving thedisplacement of oil to the production well(s). To achieve optimalefficiency in waterflooding operations, it is desirable that themobility of the water be less than the mobility of the oil. The oilmobility is calculated by the formula k_(o)/μ_(o), where k_(o) is theoil permeability and μ_(o) is the oil dynamic viscosity. Similarly, thewater mobility is calculated by k_(w)/μ_(w), where k_(w) is the waterpermeability and μ_(w) is the water dynamic viscosity. In typical waterflooding operations the water mobility is greater than the oil mobility,thus the water will tend to channel or finger through the oil. When theone or more ionic polyvinyl alcohol copolymers is added to the at leastone portion of the flooding fluid as described by aspects of the presentinvention, the addition of the one or more ionic polyvinyl alcoholcopolymers increases the viscosity of the at least one portion of thewater, thereby reducing the effective water mobility. Thus, the oil ismore likely to be driven towards the production well(s).

In one aspect, the viscosity of at least one portion of the floodingfluid comprising one or more ionic polyvinyl alcohol copolymers is about30% higher at low shear rates, wherein low shear rates are 1 sec⁻¹, orless, than the viscosity of the same polymer in solution measured at thesame temperature but at a high shear rate of 10 sec⁻¹ or greater.Consequently a figure of merit that will be used to illustrate thedegree of shear thinning is the viscosity ratio measured at a specifictemperature. Using this figure of merit, in one aspect, this viscosityratio for at least one portion of the flooding fluid comprising one ormore ionic polyvinyl alcohol copolymers is at least 1.3. In anotheraspect, this viscosity ratio of at least one portion of the floodingfluid comprising one or more ionic polyvinyl alcohol copolymers is atleast 2.5.

In a stratified oil-bearing formation the permeability of differentgeological oil-bearing layers may differ, and as a result the injectedwater could initially reach the production well through the mostpermeable layer before a substantial amount of the oil from other, lesspermeable, layers is retrieved. This breakthrough of injection water isproblematic for oil recovery, as the water/oil ratio retrieved from theproduction well will increase and become more unfavorable during thelifetime of the oil field. The addition of the one or more ionicpolyvinyl alcohol copolymers to at least one portion of the floodingfluid is expected to result in less water flooding the more permeablezones in a reservoir, thus reducing the chance of fingering of floodingfluid through these more permeable zones of the oil bearing strata andimproving sweep efficiency.

Additional materials can optionally be added as thickening agents orsurface active agents to enhance the sweep efficiency of the floodingfluid and/or reduce water mobility. These materials include at least oneof the members of the group consisting of hay, sugar cane fibers, cottonseed hull, textile fibers, shredded paper, bentonite, rubber pulp, woodshavings and nut hulls, provided that these materials together with theone or more ionic polyvinyl alcohol copolymers provide the desiredviscosity, concentration and/or particle size distribution. In addition,propanediol thickeners, such as one or more members of the groupconsisting of 1,3-propanediol; an oligomer of 1,3-propanediol; ahomopolymer of 1,3-propanediol; and a heteropolymer of 1,3-propanediol,wherein said heteropolymer is synthesized using at least one C₂ throughC₁₂ comonomer diol, as described in the commonly owned and copendingU.S. application Ser. No. 12/023,166 (page 4). An “oligomer” of1,3-propanediol has a degree of polymerization of 2-6, whereas a“polymer” has a degree of polymerization of at least 7. A “homopolymer”of 1,3-propanediol is a polymer synthesized using monomers of1,3-propanediol. A “heteropolymer” of 1,3-propanediol is a polymersynthesized using 1,3-propanediol monomers as well as one or moreadditional C₂ through C₁₂ straight-chain or branched comonomer diols.Additional thickeners include polyacrylic amide, carboxymethylcellulose,polysaccharide, polyvinyl pyrrolidone, polyacrylic, and polystyrenesulfonates, and ethylene oxide polymers, as described in U.S. Pat. No.3,757,863, column 2, line 33 to line 45; and methyl cellulose, starch,guar gum, gum tragacanth, sodium alginate, and gum arabic, as describedin U.S. Pat. No. 3,421,582, column 2, line 33 to line 45. Each of thethickeners can be used alone, or in combination with one or more otherthickeners as described above. Surfactants, such as acid salts ofamido-acids as described in U.S. Pat. No. 2,802,785, column 2, line 11to column 4, line 43 can also optionally be added. Surfactants andthickeners can also be used in combination. The use of the one or moreionic polyvinyl alcohol copolymers according to the present invention isadvantageous in that the one or more ionic polyvinyl alcohol copolymersis biodegradable and does not present environmental toxicity problems.Thus, in one aspect, the additional materials that are added to floodingfluids of the invention are preferably also biodegradable, such asstarch, guar gum, sodium alginate, gum arabic and methyl cellulose.

In one aspect, the present invention provides a method for making anaqueous flooding fluid for use in waterflooding, comprising adding oneor more ionic polyvinyl alcohol copolymers to at least one portion ofwater used in waterflooding.

The flooding fluid can be recovered as it exits the production well(s)and at least one portion of said flooding fluid can be reused, i.e.,injected into the reservoir. Prior to reinjection into the reservoir,additional one or more ionic polyvinyl alcohol copolymers as definedabove can be added to at least one portion of the recovered floodingfluid. The additional one or more ionic polyvinyl alcohol copolymers canbe added at a concentration of about 0.007% to about 3% (weight of oneor more ionic polyvinyl alcohol copolymers/weight of at least oneportion of flooding fluid). Alternatively, at least one portion of theflooding fluid exiting the production well(s) can be disposed of, forexample by disposal at sea, in a disposal well, or in a wastewater pond.

EXAMPLES

The present invention is further defined in the following Examples. Itshould be understood that these Examples, while indicating the preferredaspects of the invention, are given by way of illustration only. Fromthe above discussion and these examples, one skilled in the art canascertain the essential characteristics of this invention, and withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various uses andconditions.

The following abbreviations are used: “rpm” is revolutions per minute;“MW” is molecular weight; “mol” is mole; “L” is liter; “mL” ismilliliter; “PTFE” is polytetrafluoroethylene; “g” is gram; “mg” ismilligram; “wt %” is weight percent; “cc/min” is cubic centimeters perminute; “° C.” is degrees Centigrade or Celsius; “MHz” is Megahertz;“NMR” is nuclear magnetic resonance; and “GPC” is gel permeationchromatography, “F” means degrees fahrenheit.

Elvanol® 50-42, Elvanol® 71-30, and Elvanol® 70-62 were obtained fromDuPont de Nemours & Co., Inc. (Wilmington, Del.).

General Methods

Preparation of Copolymers

Copolymer 1 was obtained from DuPont de Nemours & Co., Inc., and was afully hydrolyzed grade of polyvinyl alcohol copolymer with 5.2 mol %methyl acrylate comonomer having a number average MW (Mn) of 36,300, andhydrolysis of 99.5%. Additional characteristics of Copolymer 1 are shownin Tables 2 and 3 below.

Copolymer 2 was prepared as described in U.S. Patent Application No.US2006/0035042, paragraph 11 through paragraph 25, and was a fullyhydrolyzed grade of polyvinyl alcohol copolymer with 3.0 mol % methylacrylate comonomer and 3.0 mol % itaconic acid comonomer having a numberaverage MW (Mn) of 41,600 and hydrolysis of 99.3%. Additionalcharacteristics of Copolymer 2 are shown in Tables 2 and 3 below.

TABLE 1 Preparation conditions for copolymers 3 through 6 Copolymer 3 45 6 AA Precharge — — 1.4 2.2 (g) IA Precharge 1.54 1.54 — — (g) MAPrecharge 1.62 1.62 1.1 1.8 (g) AA Feed rate — — 0.086 0.138 (cc/min) IAFeed rate 0.468 0.491 — — (cc/min) MA Feed rate 0.073 0.076 0.076 0.122(cc/min) Polym. Time 85 75 45 40 (min.) g HQ/ 0/2/80 0.2/4/130 0.2/4/1000.2/4/100 NaNO₂/g MeOH final solids (%) 20.7 17.8 23.4 Not determinedCopolymers 3 through 6 were prepared as follows: (modified from AD7167USNA Spec). A 2 L polymer kettle was assembled with an overhead stirrer,a Claisen head with a multi-inlet port fitting connected to 2 syringepumps and a tap-H₂O condenser further attached to a N₂ bubbler, anoverhead thermowell with a thermocouple device, and a septum. Theagitator was a single PTFE paddle and the stir rate was set to 150-185rpm. The kettle was charged with 1000 g vinyl acetate (Aldrich Chemicalco., Milwaukee, Wis., USA) and 11 g methanol (EM Science). Itaconic acid(IA, Aldrich), methyl acrylate (MA, Aldrich), and acrylic acid (AA,Aldrich) were then charged to the kettle in the amount specified inTable 1. The mixture was degassed at room temperature for 20 minuteswith a sparging tube. The kettle was then heated with a 77-85° C. oilbath. One syringe pump was charged with neat methyl acrylate. A secondsyringe pump was charged with neat acrylic acid or a 24.7 wt % solutionof itaconic acid in MeOH. 2 g of VAZO-64 (DuPont, Wilmington, Del., USA)dissolved in 100 g of MeOH was added to the reactor. The two syringepumps were then activated to feed at the rates specified in the table.The polymerization was allowed to proceed for the time indicated in thetable then a mixture of hydroquinone (HQ, Aldrich), sodium nitrite(Aldrich) and MeOH were added all at once to halt the polymerization.The solids were determined by average of 3 samples of about 0.5 g weightthat were dried 18 hour in a 80° C. vacuum oven. The polyvinyl acetateterpolymer was transferred to a 3 L round bottom flask and theMeOH/vinyl acetate azeotrope was removed under reduced pressure.Addition of 500 g of MeOH followed by azeotropic distillation wasrepeated 3 times, at which time removal of vinyl acetate was judgedsubstantially complete.

TABLE 2 Characteristics of copolymers 4% Solubility Aqueous GPC GPC inSea Solution Methyl Itaconic Acrylic Number Weight Water Viscosity %Acrylate Acid Acid Average Average Simulant PVOH (cP) Hydrolysis (mol %)(mol %) (mol %) MW MW at 45-46° F. Copolymer 3 72 (b) 99.2% (c) 3.0 2.873,700 171,800 50% soluble @ 120 min. Elvanol 27.0-33.0 (a) 99.0-99.8(a) — — 46,400 96,000 (e) 71-30 Elvanol 44.0-50.0 (a) 87.0-89.0 (a) — —73,580 132,800 50% 50-42 soluble @ 120 min. Elvanol 58.0-68.0 (a)99.4-99.8 (a) — — 55,800 148,000 (e) 70-62 Copolymer 1 20 (b) 99.5% (d)5.2 — 36,300 70,900 (e) Copolymer 2 19 (b) 99.3% (c) 3.0 3.0 41,60077,700 10 min. Copolymer 4 64 (b) 99.1% (c) 3.2 2.9 88,750 202,800 95%soluble in 90 min. Copolymer 5  99 100 (c) 1.6 2.3 99,900 199,100 0%soluble in 120 min. Copolymer 6 105   99.8 (c) 2.7 3.9 98,900 256,800100% dissolved 50 min. (a) manufacturer specification (b) measured value(c) measured by ATR method (d) measured by NMR method (e) insoluble incold water, so not subjected to the test

The polyvinyl acetate was converted to polyvinyl alcohol bytransesterification with methanol and sodium methoxide. The polyvinylacetate was divided into two equal portions and subjected to the sameprocedure. Polyvinyl acetate dissolved in MeOH was charged to a 1 gallonexplosion-proof stainless steel blender (Eberbach Corp., Ann Arbor,Mich.). The blender was set to stir at 10,000 rpm then 1.5 g of 25 wt %sodium methoxide in methanol (Aldrich) per gram of polyvinyl acetate wasadded through a small hole in the blender's cover. The ensuingheterogeneous mixture was stirred for 10 minutes, then 1.2 molarequivalents of glacial acetic acid (EM Science) relative to sodiummethoxide were slowly added. The mixture was stirred 2 minutes, thenfiltered. The polymer product was rinsed 4 times with MeOH, then driedin a vacuum oven at 80° C. overnight.

The compositions of Copolymers 3, 4, and 5, Table 2, were determined bymass balance analysis assuming the acrylic acid, methyl acrylate anditaconic acid were 100% incorporated into the terpolymer. NMR analysisconfirmed 3.8 mol % total acrylic acid and methyl acrylate comonomercontent in Copolymer 5, which is in good agreement with the mass balancecomposition determination of 3.9 mol %. The NMR analysis was used todetermine that Copolymer 6 was 6.6 mol % total comonomers. The AA:MAratio was 59.6:40.4 therefore a comonomer loading of 3.9 mol % AA and2.7 mol % MA was calculated.

The polymers used in Examples 1-16 are characterized in Table 2 and 3.In Table 2, the 4% Aqueous solution viscosity was measured with afalling ball or falling needle viscometer as is well known in the art(Polyvinyl Alcohol Properties and Applications, ed. by C. A. Finch,Wiley, New York, 1973, p 570) % Hydrolysis values were determined by ¹HNMR analysis at 80° C. at 400 MHz (Bruker, Billerica, Mass.) in D₂O orby the attenuated total reflection infrared (ATR) method (SmithDetection Scientific, Danbury, Conn.). The D₂O was 100% grade fromCambridge Isotope Laboratories, Inc., (Andover, Mass.). As confirmed byATR analysis, a series of polyvinyl alcohol samples with % hydrolysisvalues ranging from 86.6-99.6% as determined using an aqueoussaponification method (as described in Finch, C. A., (ed) PolyvinylAlcohol-Developments (John Wiley & Sons, New York, 1992, page 754) weresynthesized.

A ratio of the acetate methyl absorption at 1273 cm⁻¹ to the 844 cm⁻¹PVOH methylene peak was determined. A calibration curve was then plottedusing the calculated ratios, and the calibration curve was used todetermine the % hydrolysis values of the polyvinyl alcohol samples. GPCdata were acquired using a 150CV system from Waters Corporation(Milford, Mass.) according to the manufacturer recommendations withdimethyl sulfoxide (DMSO), dimethyl acetamide (DMAc), orhexafluoroisopropanol mobile phases. Solubility was assessed by charginga 20 mL vial equipped with a magnetic stir bar and cap with 50 mg ofpolyvinyl alcohol and 3 mL of water and stirring at 22-25° C. Visualobservations of solubility were made periodically.

An additional test method was employed to determine the comonomercontent of the Acrylic Acid terpolymers for copolymers 5 and 6. 10 mg ofthe sample was dissolved in D₂O (100% grade, Cambridge) after thoroughmixing using a Vortex mixer. The ¹H NMR spectrum was acquired in aBruker 500 MHz NMR spectrometer with a 5 mm BBIz probe (228) at 80° C.,acquisition time 4.68 s, a 90 degree pulse of 8.15 microseconds and arecycle delay of 30 s. 16 scans were acquired. Immediately after, thesample was removed from the magnet, cooled down for a few minutes andthen 3.6 μL of NaOD was added (CAS 14014-06-3 Product number: 37,207-2,Aldrich Chemical Co.)

TABLE 3 Summary of viscosity ratio of the polymers tested. Wt % ofViscosity Viscosity Viscosity Polymer or polymer in Ratio* at Ratio* atRatio* at Example coplymer solution at 25 C. 55 C. 80 C. Dissolveseasily? 1 3 0.1 in fresh 1.4 1.0 14.7 No (fresh water) water 2 3 0.1 1.51.8 2.7 No (sea water) 3 3 0.5 1.7 2.1 8.9 No (sea water) 4 Elvanol ®0.1 2.7 2.6 7.8 No 71-30 5 Elvanol ® 0.1 2.5 0.9 3.0 No 50-42 6Elvanol ® 0.1 1.1 6.6 304.2 No 70-62 7 1 0.1 2.0 2.4 2.5 No 8 2 0.1 0.80.8 1.7 Yes 9 2 1.0 0.9 1.7 1.5 Yes 10 4 0.1 3.6 3.7 5.1 yes 11 4 0.31.4 2.5 3.5 yes 12 5 0.1 2.3 1.3 4.0 No 13 5 1.0 1.0 1.1 1.1 No 14 6 0.12.6 3.2 6.2 Yes 15 6 1.0 1.6 2.5 3.5 yes *Ratio of the viscositymeasured at that temperature and at a shear rate of 1 sec⁻¹ to theviscosity measured at that temperature and at a shear rate of 10 sec⁻¹

In Table 3, the ratio of viscosity was determined from viscositymeasurements taken as a function of shear rate using a Brookfield DV-II+Pro instrument (Brookfield Engineering Laboratories, Inc., Middleboro,Mass.) using a UL adaptor with water jacketed cup and remote temperaturedetection probe. The instrument was controlled using Rheocal softwarev2.7. The shear rate was varied from 0.25 sec⁻¹ to 250 sec⁻¹ at 25, 55and 80° C. Values of viscosity at a shear rate of 1 sec⁻¹ and 10 sec⁻¹were used in the calculation of the viscosity ratio. This viscosityratio was measured at the various temperatures to match the likely rangein the reservoir temperature. Most reservoirs are warm but cool downnear the injector well under prolonged water flood. Near the injectorwell bore, the temperatures are near 25° C. As the water moves out awayfrom the injector to the producer, temperatures rise and the amount ofoil that is likely to be left in the formation also increases asdistance away from the injector to the producer well increases. Hence itis desirable to have a high viscosity ratio at a high temperature, e.g.,80° C.

Example 1 Behavior of Copolymer 3 in Deionized Water

Copolymer 3 was dissolved in deionized water to a concentration of 0.1weight percent. It took overnight agitation at room temperature todissolve the polymer. The viscosity was measured as described above andthe viscosity ratio calculated and is presented in Table 3. Althoughthis polymer did not quickly dissolve in deionized water, the viscosityratio showed significant thickening at the highest temperature, e.g.,80° C. Consequently this polymer system would work for polymer floodingapplications but only if more time or warmer water were used todissolved the polymer.

Example 2 Behavior of Copolymer 3 in Sea Water at 0.1 Weight Percent

Copolymer 3 was dissolved in synthetic sea water to a concentration of0.1 weight percent. Synthetic sea water was acquired from EMD ChemicalsInc, (Gibbstown N.J., Part number GC0118/1, lot#7050). Although iteventually completely dissolved, it was only 50% dissolved in syntheticseawater at 45° F. after 120 minutes. The viscosity and sheer rate weremeasured and the raw data smoothed by taking a 3 point running average.The viscosity ratio is shown in Table 3. The viscosity ratio forcopolymer 3 did show improved with increasing temperature and showed hada value of 2.7 at 80° C. Consequently this polymer system would work forpolymer flooding applications but only if more time or warmer water wasused to dissolved the polymer.

Example 3 Behavior of Copolymer 3 in Sea Water at 0.5 Weight Percent

Copolymer 3 was dissolved in synthetic sea water to a concentration of0.5 weight percent. Synthetic sea water was acquired from EMD ChemicalsInc, Gibbstown N.J., Part number GC0118/1, lot#7050. Although iteventually completely dissolved, it was only 50% dissolved in syntheticseawater at 45° F. after 120 minutes. The viscosity and sheer rate weremeasured and the raw data smootherd as described above. The viscosityratio is shown in Table 3. The viscosity ratio for this higherconcentration of copolymer 3 increased with increasing temperature andhad a greater ratio at this higher concentration compared to Example 2.It had a viscosity ratio value of 8.9 at 80° C. Consequently thispolymer system, at this higher concentration, would work for polymerflooding applications but only if more time or warmer water were used todissolved the polymer.

Example 4 Effect of Continuous Agitation at High Temperature onDissolution of Dupont Elvanol® 71-30

DuPont Elvanol® 71-30 polymer was dissolved in synthetic sea water to aconcentration of 0.1 weight percent. Dissolution required continuousagitation and heating at 80° C. to 90° C. for a day. The viscosity andsheer rate of this solution were measured and the raw data smoothed asdescribed above. The viscosity ratio is shown in Table 3. The viscosityratio, with a value of 7.8 at 80° C., had increased with increasingtemperature. Consequently this polymer system, at this higherconcentration, would work for polymer flooding applications but only ifmore time or warmer water were used to dissolved the polymer.

Example 5 Behavior of DuPont Elvanol® 50-42 in Sea Water at 0.1 WeightPercent

DuPont Elvanol® 50-42 homopolymer was dissolved in synthetic sea waterto a concentration of 0.1 weight percent. Dissolution requiredcontinuous agitation and heating at 80° C. to 90° C. for about a day.The viscosity and sheer rate were measured and the raw data smoothed asdescribed above. The viscosity ratio is shown in Table 3. The viscosityratio favorable at 25° C. and 80° C. but not at 55° C. The viscosityratio was not as great as many of the other polymer systems tested inTable 3. Consequently this polymer system would work but not as well asother tested for polymer flooding applications but only if more time orwarmer water were used to dissolved the polymer

Example 6 Behavior of DuPont Elvanol® 70-62 in Sea Water at 0.1 WeightPercent

DuPont Elvanol® 70-62 homopolymer was dissolved in synthetic sea waterto a concentration of 0.1 weight percent. Dissolution requiredcontinuous agitation and heating at 80 to 90 C for about a day. Theviscosity and sheer rate were measured and the raw data smoothed asdescribed above. The viscosity ratio is shown in Table 3. The viscosityratio was observed to increase with increasing temperature and had aremarkably high viscosity ratio value of 304 at 80° C. Consequently thispolymer system would work for polymer flooding applications but only ifmore time or warmer water were used to dissolved the polymer.

Example 7 Behavior of Copolymer 1 in Sea Water at 0.1 Weight Percent

Copolymer 1 was dissolved in synthetic sea water to a concentration of0.1 weight percent. Dissolution required continuous agitation andheating at 80° C. to 90° C. for about a day. The viscosity and sheerrate were measured and the raw data smoothed as described above Theviscosity ratio is shown in Table 3. Although viscosity ratio isfavorable, its ratio was not as significant as some of the other polymersystems tested in Table 3. Consequently this polymer system would workbut not as well as other tested for polymer flooding applications butonly if more time or warmer water was used to dissolved the polymer

Example 8 Behavior of Copolymer 2 in Sea Water at 0.1 Weight Percent

Copolymer 2 was dissolved in synthetic sea water to a concentration of0.1 weight percent. The polymer readily dissolved in cold synthetic seawater within a few minutes as indicated in Table 2. The viscosity andsheer rate were measured and the raw data smoothed as described above.The viscosity ratio is shown in Table 3. The viscosity ratio is onlyslightly favorable at 80° C. This polymer system at this concentrationhad the poorest viscosity ratios. This example illustrates that thecompositional changes needed to make a polymer soluble (low molecularweight in this case) will lead to poor shear thinning. Consequently thispolymer system would not work well in polymer flooding applicationsdespite being readily dissolvable.

Example 9 Behavior of Copolymer 2 in Sea Water at 1.0 Weight Percent

Copolymer 2 was dissolved in synthetic sea water to a concentration of 1weight percent. This was an attempt to see if higher concentrations ofcopolymer 2 would results in higher viscosity ratios. The polymerreadily dissolved in cold synthetic sea water within a few minutes asindicated in Table 2. The viscosity and sheer rate were measured and theraw data smoothed as described above. The viscosity ratio is shown inTable 3. The viscosity ratio is only slightly favorable at 80° C. Thispolymer system at this concentration had viscosity ratios comparable toexample 8. This example illustrates, in this case, higher concentrationshad minimal improvements in viscosity ratio. Consequently, as in thecase in Example 8, this polymer system would not work well in polymerflooding applications despite being readily dissolvable.

Example 10 Behavior of Copolymer 4 in Sea Water at 0.1 Weight Percent

Copolymer 4 was dissolved in synthetic sea water to a concentration of0.1 weight percent. The polymer dissolved in cold synthetic sea waterwithin 90 minutes as indicated in Table 2. The viscosity and sheer ratewere measured and the raw data smoothed as described above. Theviscosity ratio is shown in Table 3. The viscosity ratio is favorable atall temperatures and is substantially higher than the copolymer 2(Examples 8 and 9). Copolymers 4 and 2 are nearly the same compositionbut Copolymer 4 has a substantially higher molecular weight. Whencompared to Examples 8 and 9, the higher molecular weight givesfavorable viscosity ratios and a longer but favorable dissolution time.Consequently, this polymer system would work well in polymer floodingapplications and is also dissolvable in cold sea water.

Example 11 Behavior of Copolymer 4 in Sea Water at 0.3 Weight Percent

Copolymer 4 was dissolved in synthetic sea water to a concentration of0.3 weight percent. This was an attempt to see if higher concentrationsof copolymer 4 would results in higher viscosity ratios. The polymerdissolved in cold synthetic sea water within 90 minutes as indicated inTable 2. The viscosity and sheer rate were measured and the raw datasmoothed as described above The viscosity ratio is shown in Table 3. Theviscosity ratio is favorable at all temperatures and is substantiallyhigher than the copolymer 2 (Examples 8 and 9). However, when comparedto Example 10 the higher concentration of this copolymer did not lead tohigher viscosity ratios although these ratios are still favorable.Consequently, this polymer system would work well in polymer floodingapplications and is also dissolvable in cold sea water.

Example 12 Behavior of Copolymer 5 in Sea Water at 0.1 Weight Percent

Copolymer 5 was dissolved in synthetic sea water to a concentration of0.1 weight percent. Although it eventually completely dissolved, nonewas dissolved in synthetic seawater at 45° F. after 120 minutes. Theviscosity and sheer rate were measured and the raw data smoothed asdescribed above. The viscosity ratio is shown in Table 3. The viscosityratio is favorable at all temperatures and is substantially higher thanthe copolymer 2 at a comparable concentration (Example 8) but not asgood as Copolymer 4 (Example 10). This illustrates the effect ofchanging the comonomer composition. This polymer system would work inpolymer flooding applications but is not readily dissolvable in cold seawater.

Example 13 Behavior of Copolymer 5 in Sea Water at 1.0 Weight Percent

Copolymer 5 was dissolved in synthetic sea water to a concentration of 1weight percent. Although it eventually completely dissolved, none wasdissolved in synthetic seawater at 45° F. after 120 minutes. Theviscosity and sheer rate were measured and the raw data smoothed asdescribed above. The viscosity ratio is shown in Table 3. Remarkably,the viscosity ratio is not favorable at all temperature. Thisillustrates that a higher concentration in solutions (as compared toExample 12) does not necessarily translate to a higher viscosity ratio.This polymer system, at this higher concentration, would not work inpolymer flooding applications and is not readily dissolvable in cold seawater.

Example 14 Behavior of Copolymer 6 in Sea Water at 0.1 Weight Percent

Copolymer 6 was dissolved in synthetic sea water to a concentration of0.1 weight percent. The polymer dissolved in cold synthetic sea waterwithin 50 minutes as indicated in Table 2. The viscosity and sheer ratewere measured and the raw data smoothed as described above The viscosityratio is shown in Table 3. The viscosity ratio is favorable at alltemperatures and is substantially higher than the copolymer 5 at acomparable concentration (Example 12) and is comparable to Copolymer 4(Example 10). This illustrates the effect of changing the comonomercomposition while keeping about the same molecular weight of thepolymer. This polymer system would work in polymer flooding applicationsand is readily dissolvable in cold sea water.

Example 15 Behavior of Copolymer 6 in Sea Water at 1.0 Weight Percent

Copolymer 6 was dissolved in synthetic sea water to a concentration of 1weight percent. The polymer dissolved in cold synthetic sea water within50 minutes as indicated in Table 2. The viscosity and sheer rate weremeasured and the raw data smoothed as described above. The viscosityratio is shown in Table 3. The viscosity ratio is favorable at alltemperatures and is substantially higher than the copolymer 5 at acomparable concentration (Example 13) and is comparable to Copolymer 4(Example 11). This illustrates the effect of changing the comonomercomposition while keeping about the same molecular weight of thepolymer. When compared to Example 14, it also illustrates that a higherpolymer concentration in solution does not lead to higher viscosityratios. This polymer system would work in polymer flooding applicationsand is readily dissolvable in cold sea water.

1. A method for recovering oil from a reservoir by waterflooding,comprising: (a) introducing an aqueous flooding fluid into thereservoir, wherein at least one portion of said flooding fluid comprisesvinyl alcohol and vinyl acetate and one or more ionic polyvinyl alcoholcopolymers, said one or more ionic polyvinyl alcohol copolymerscomprising: (i) one or more anionic comonomers present at a total ofabout 1 to about 5 mol % relative to the combined moles of vinyl alcoholand vinyl acetate; (ii) optionally, one or more non-ionic comonomerspresent at about 0 to about 7 mol % relative to the combined moles ofvinyl alcohol and vinyl acetate; wherein the total monomers in (i) and(ii) are present at less than or equal to about 8 mol %; wherein thehydrolysis level of the one or more ionic polyvinyl alcohol copolymersis greater than or equal to about 90%; and wherein the one or more ionicpolyvinyl alcohol copolymers dissolves substantially completely in waterat about 25° C. within about 14 hours; and (b) displacing oil in thereservoir with said flooding fluid into one or more production wells,whereby the oil is recoverable.
 2. The method of claim 1, wherein saidone or more anionic monomers is selected from the group consisting ofitaconic acid, one or more C₁ to C₄ straight-chain or branched alkylmonoesters of itaconic acid, maleic acid, one or more C₁ to C₄straight-chain or branched alkyl monoesters of maleic acid, sodium2-acrylamido-2-methyl-1-propanesulfonate, acrylic acid or methacrylicacid.
 3. The method of claim 2, wherein said one or more anionicmonomers is itaconic acid or one or more C₁ to C₄ straight-chain orbranched alkyl monoesters of itaconic acid.
 4. The method of claim 2,wherein said one or more anionic monomers is sodium2-acrylamido-2-methyl-1-propanesulfonate.
 5. The method of claim 2,wherein said one or more anionic monomers is maleic acid or one or moreC₁ to C₄ straight-chain or branched alkyl monoesters of maleic acid. 6.The method of claim 1, wherein said one or more non-ionic comonomers isone or more C₁ to C₄ straight-chain or branched alkyl esters of acrylicor methacrylic acid.
 7. The method of claim 1, wherein said one or morenon-ionic comonomers is selected from the group consisting of ethylene,acrylamide and vinyl pyrrolidone.
 8. The method of claim 1, wherein saidone or more non-ionic comonomers is methyl acrylate.
 9. The method ofclaim 1, wherein the hydrolysis level of the one or more ionic polyvinylalcohol copolymers is greater than or equal to about 95%.
 10. The methodof claim 9, wherein the hydrolysis level of the one or more ionicpolyvinyl alcohol copolymers is greater than or equal to about 98%. 11.The method of claim 10, wherein the hydrolysis level of the one or morepolyvinyl alcohol copolymers is greater than or equal to about 99%. 12.The method of claim 11, wherein said one or more anionic monomers isitaconic acid and said one or more non-ionic comonomers is methylacrylate.
 13. The method of claim 9, wherein said one or more anionicmonomers is sodium 2-acrylamido-2-methyl-1-propanesulfonate.
 14. Themethod of claim 9, wherein said one or more anionic monomers is itaconicacid or one or more C₁ to C₄ straight-chain or branched alkyl monoestersof itaconic acid.
 15. The method of claim 9, wherein said one or moreanionic monomers is maleic acid or one or more C₁ to C₄ straight-chainor branched alkyl monoesters of maleic acid.
 16. The method of claim 1,wherein the one or more ionic polyvinyl alcohol copolymers has a numberaverage molecular weight greater than about 50,000 daltons as measuredby gel permeation chromatography.
 17. The method of claim 1, wherein theone or more ionic polyvinyl alcohol copolymers has a number averagemolecular weight greater than about 60,000 daltons as measured by gelpermeation chromatography.
 18. The method of claim 1, wherein the one ormore ionic polyvinyl alcohol copolymers has a number average molecularweight greater than about 70,000 daltons as measured by gel permeationchromatography.
 19. The method of claim 1, wherein said flooding fluidis recovered, and wherein at least one portion of said the recoveredflooding fluid is reinjected into the reservoir.
 20. The method of claim19, wherein said recovered flooding fluid is supplemented with one ormore ionic polyvinyl alcohol copolymers, said one or more ionicpolyvinyl alcohol copolymers dissolves substantially completely in waterat about 25° C. within about 14 hours; prior to reinjection.
 21. Themethod of claim 1 or claim 20, wherein, said one or more ionic polyvinylalcohol copolymers are added at a concentration of about 0.007% to about3% by weight relative to the weight of the at least one portion offlooding fluid.
 22. The method of claim 1, wherein said flooding fluidis disposed of.
 23. The method of claim 22, wherein the flooding fluidis disposed of at sea, in a disposal well, or in a wastewater pond. 24.The method of claim 1, wherein said aqueous flooding fluid comprises seawater, brine, production water, water recovered from an undergroundaquifer, or surface water from a stream, river, pond or lake.
 25. Themethod of claim 1, wherein the total monomers in (i) and (ii) arepresent at less than or equal to about 8 mol %; wherein the hydrolysislevel of the one or more ionic polyvinyl alcohol copolymers is greaterthan or equal to about 90%; and wherein the one or more ionic polyvinylalcohol copolymers dissolves substantially completely in water at about25° C. within about 14 hours; increase the shear thinning properties ofthe flooding fluid.
 26. The method of claim 1, wherein the at least oneportion of the flooding fluid exhibits a low viscosity during injectioninto the reservoir and a higher viscosity when flowing through thereservoir.
 27. The method of claim 26, wherein the viscosity ratio ofthe at least one portion of the flooding fluid comprising polymer is atleast 1.3 or at least 2.5.
 28. The method of claim 1, wherein theaqueous flooding fluid further comprises at least one of the groupconsisting of hay, sugar cane fibers, cotton seed hulls, textile fibers,shredded paper, bentonite, rubber pulp, wood shavings, nut hulls,polyacrylic amide, carboxymethylcellulose, polysaccharide, polyvinylalcohol, polyvinyl pyrrolidone, polyacrylic, polystyrene sulfonates,ethylene oxide polymers, methyl cellulose, starch, guar gum, gumtragacanth, sodium alginate, gum Arabic and surfactants.
 29. The methodof claim 28, wherein the aqueous flooding fluid further comprisesstarch, guar gum, sodium alginate, gum arabic or methyl cellulose. 30.The method of claim 1, wherein the aqueous flooding fluid furthercomprises one or more members selected from the group consisting of1,3-propanediol; an oligomer of 1,3-propanediol; a homopolymer of1,3-propanediol; and a heteropolymer of 1,3-propanediol, wherein saidheteropolymer is synthesized using at least one C₂ through C₁₂ comonomerdiol.
 31. The method of claim 1, wherein the sweep efficiency isimproved.
 32. The method of claim 1, wherein the mobility ratio improvesand becomes more favorable to mobilizing oil.